This invention relates generally to fluid control valves and, more particularly, to fluid control valve structures which provide highly controllable flow characteristics and which limit sliding seal friction during valve plug travel.
One type of prior art fluid control valve includes a valve body having inlet and outlet ports, a sleeve-like cage, a valve plug that is slidable within the cage and a bonnet assembly which retains the cage and the valve plug in the valve body. The inlet port communicates with the outlet port through apertures, or windows, in the cage. Fluid flow through the valve is determined by the position of the valve plug relative to the windows in the cage. An example of a prior art fluid control valve is disclosed in U.S. Pat. No. 3,776,278 issued Dec. 4, 1973 to Allen.
Prior art fluid control valves have provided satisfactory performance but have certain drawbacks. The flow characteristics may be difficult to control, particularly near the off position of the valve plug. In addition, the valve may generate undesirable noise under certain flow conditions. Furthermore, the seating surface of the valve plug may be subject to erosive wear over time, thereby degrading valve operation. Frictional forces encountered in changing the position of the valve plug may cause control systems to be unstable. Component tolerances and thermal expansion may produce an unsatisfactory fit between valve parts and may degrade performance. Conventional gate valves are limited in fluid flow capacity for a given orifice diameter.
Accordingly, there is a need for fluid control valve structures which overcome one or more of the above drawbacks.
According to a first aspect of the invention, a fluid control valve is provided. The fluid control valve comprises a valve body having a fluid passageway, a cage mounted in the fluid passageway, and a valve plug assembly including a valve plug. The cage includes a cage bore having an axis. The cage defines one or more windows, a valve seat located below the windows and a sealing lip located above the windows. The valve plug is axially slidable in the cage bore. The valve plug has one or more pressure balancing passages between opposite ends thereof and a seating surface for engaging the valve seat. The valve plug assembly further comprises an elastically deformable sealing ring for engaging the sealing lip when the valve plug is in a closed position.
The deformable sealing ring may comprise an elastic metal. Preferably, the deformable sealing ring contacts the cage only in the closed position and near the closed position. The cage bore may have a larger diameter above the sealing lip than below the sealing lip. The valve plug assembly may further comprise an elastomer ring located below the deformable sealing ring.
According to another aspect of the invention, a fluid control valve is provided. The fluid control valve comprises a valve body having a fluid passageway, a cage mounted in the fluid passageway, and a valve plug assembly including a valve plug. The cage includes a cage bore having an axis. The cage defines one or more windows, a valve seat located below the windows and a flow control portion of the cage bore located between the valve seat and the windows. The flow control portion has an interior contour that varies in dimension as a function of distance from the valve seat. The valve plug is axially slidable in the cage bore between a closed position in engagement with the valve seat, a partially open position wherein fluid flow is determined by a spacing between the valve plug and the flow control portion of the cage bore, and a more open position wherein fluid flow is determined by the windows and by the spacing between the valve plug and the flow control portion.
In a first embodiment, the interior contour of the flow control portion has a diameter that is an exponential function of distance from the valve seat. In a second embodiment, the interior contour of the flow control portion has a diameter that is a linear function of distance from the valve seat. The interior contour of the flow control portion may be selected to provide a desired flow characteristic.
The valve plug may include a cylindrical section that defines a spacing between the valve plug and the flow control portion of the cage bore, and an annular expansion groove above the cylindrical section. The annular expansion groove may have a surface that slopes away from the cylindrical section at an angle of at least 30xc2x0 with respect to the cage bore axis.
The valve plug may include a seating surface for engaging the valve seat and an extension below the seating surface. The extension may be shaped to provide initial throttling of the fluid, deflecting fluid flow away from the seating surface and thereby limiting erosive wear caused by high fluid velocity and entrained particles. The valve plug may further include a streamlined, curved contour below the extension for guiding the fluid smoothly into the windows when the valve is at or near the open position. This is further aided by streamlined entrance portions of the cage windows.
The valve plug may include one or more pressures balancing passages between opposite ends thereof. The cage bore may include a sealing lip located above the windows, and the valve plug assembly may include an elastically deformable sealing ring for engaging the sealing lip in the closed position.
The cage may be constructed to be axially deformable. The axially deformable cage compensates for machining tolerances and thermal expansion.